Project Summary Pathological substance use disorders are a public health crisis leading to tremendous morbidity and mortality for afflicted patients and incalculable costs to society at large. Addiction to cocaine and other psychostimulants accounts for a significant proportion of this burden of disease, and treatment of these patients is currently limited by the lack of any FDA-approved pharmacotherapies. Despite significant advances in our understanding of the dopaminergic, glutamatergic, and intracellular signaling cascades altered in models of stimulant use disorders, efforts to develop medications aimed at treating stimulant use disorder have been unsuccessful. There is a growing appreciation for the role of neuroimmune interactions in normal brain function and plasticity as well as in the pathophysiology of neuropsychiatric diseases. Microglia, the resident immune cells of the CNS, interact with neurons, prune synapses, and produce neurotrophic factors that can alter synaptic plasticity and behavior. We have recently identified granulocyte-colony stimulating factor (G-CSF) as a cytokine that is increased in blood and brain following prolonged cocaine. Systemic injections of G-CSF enhance the formation of conditioned place preference and enhance motivation to self-administer cocaine. Additionally, G-CSF potentiates cocaine induction of the immediate early gene c-Fos and enhances dopamine release from the ventral tegmental area into the nucleus accumbens (NAc). Interestingly, the receptor for G- CSF is expressed exclusively on microglia in the NAc. In this proposal we will utilize cutting-edge in vivo imaging technology to directly visualize and interrogate the effects of this microglial modulator on patterns of neuronal activity that encode cocaine administration and seeking. In Aim 1 we will record calcium signals in D1 and D2 expressing medium spiny neurons in the NAc of animals treated with G-CSF or vehicle during active cocaine self-administration or during a drug seeking task. Given that the D1 and D2 expressing populations of neurons have been shown to have opposing effects on encoding rewarding stimuli, these experiments will provide crucial information as to how G-CSF is shifting the balance of patterns of neural activity between these two discrete cell populations. In Aim 2, we will test the causal nature of G-CSF signaling through microglia by using a transgenic mouse model that deletes the G-CSF receptor exclusively in microglia and measure behavioral and neural circuit changes. Together, these experiments will characterize the neural circuit changes induced by G-CSF signaling through microglia and elucidate the mechanisms by which microglial signaling controls cocaine-associated behavior.